This invention relates to improvements in an air-fuel ratio control system for an internal combustion engine, and more particularly to a technique for controlling the air-fuel ratio for the purpose of improving a NOx emission control performance.
Hitherto, it has been known that an automotive internal combustion engine is equipped with a NOx trap catalyst which is arranged to trap NOx when the air-fuel ratio of exhaust gas flowing to the NOx trap catalyst is lean and to release and reduce the trapped NOx when the air-fuel ratio of exhaust gas is rich. Accordingly, after the engine is operated at a lean air-fuel ratio, the engine is operated at a rich air-fuel ratio thereby reducing NOx. In case of enriching the air-fuel ratio in order to reduce NOx, insufficient reduction of NOx can be made if a time for enriching the air-fuel ratio is insufficient, thereby increasing the emission level of NOx. In contrast, if the time for enriching the air-fuel ratio is too long so that excessive fuel is injected from a fuel injector, an emission level of CO and HC (hydrocarbons) increases.
In view of the above, it is required in exhaust gas emission control to accomplish an optimum enrichment for the air-fuel ratio for the purpose of preventing an increase in emission level of CO and HC while sufficiently reducing NOx. In order to meet such purpose, some proposals have been made as disclosed in Japanese Patent Provisional Publication Nos. 8-232646 and 11-210524. According to such proposals, the enrichment of the air-fuel ratio is terminated at a timing when the air-fuel ratio detected by an air-fuel ratio sensor disposed downstream of the NOx trap catalyst shifts to the stoichiometric value or to a value richer than the stoichiometric value, thereby intending accomplishing the optimum enrichment.
It will be understood that the NOx trap catalyst and other catalysts in the exhaust gas passageway of the engine store therein oxygen contained in exhaust gas. Consequently, in case that NOx is trapped in the NOx trap catalyst when the air-fuel ratio of exhaust gas is lean, a major part of CO and HC in enriched exhaust gas for NOx reducing purpose is used to consume oxygen stored in the catalysts.
In order to accomplish such an optimum enrichment of the air-fuel ratio as to sufficiently reduce NOx and not to increase the emission level of CO and HC, it is required to make the enrichment of the air-fuel ratio corresponding to the amount of oxygen stored in the catalysts in the exhaust gas passageway.
Oxygen stored in the catalyst disposed in the exhaust gas passageway when the air-fuel ratio is lean will be released from the catalysts when the air-fuel ratio is rich to react with CO and HC in exhaust gas. However, the rate of releasing oxygen is not constant and lowers as the amount of oxygen remaining in the catalyst decreases, in which it is assumed that the oxygen concentration in exhaust gas at the position of the outlet of the catalyst is lowered. In this regard, experiments of the present inventors have revealed that, in practice, oxygen remains in the catalysts even when the air-fuel ratio sensor disposed downstream of the catalyst has judged that the air-fuel ratio becomes rich after the air-fuel ratio of exhaust gas flowing to the catalyst is enriched or become rich. Accordingly, if the enrichment of the air-fuel ratio of exhaust gas flowing to the catalyst is terminated at the timing of the judgment by the air-fuel ratio sensor, the enrichment of the air-fuel ratio becomes insufficient.
In the conventional techniques disclosed in Japanese Patent Provisional Publication Nos. 8-232646 and 11-210524, the enrichment of the air-fuel ratio is terminated at the timing when the air-fuel ratio detected by the air-fuel ratio sensor disposed downstream of the catalyst has shifted to the stoichiometric level or to a level richer than the stoichiometric level. As a result, it is impossible to judge as to whether oxygen stored in the catalysts has been completely released or not. Thus, NOx cannot be effectively reduced in such a conventional manner as to terminate the enrichment of the air-fuel ratio at the timing when the air-fuel ratio detected by the air-fuel ratio sensor disposed downstream of the catalyst has shirted to the stoichiometric level or the level richer than the stoichiometric level.
Particularly in case that shifting is made to a feedback control for the stoichiometric air-fuel ratio under the action of the air-fuel ratio sensor disposed upstream of the catalyst after a rich air-fuel ratio engine operation is made, the catalyst is lowered in NOx reduction ability thereby increasing the emission level of NOx. Because, oxygen has still remained in the catalysts in the above conventional manner to terminate the enrichment of the air-fuel ratio at the timing when the air-fuel ratio detected by the air-fuel ratio sensor disposed downstream of the catalyst shifts to the stoichiometric level or the level richer than the stoichiometric level.
Additionally, efficiencies of releasing and reducing NOx upon reacting with CO and HC increase as the amount of oxygen stored in the catalyst decreases. Accordingly, in case that oxygen has remained in the catalysts, NOx tends to remain in the NOx trap catalyst as compared with another case in which oxygen has been completely released from the NOx trap catalyst. Therefore, even in case that returning is made to the lean air-fuel ratio engine operation immediately after the rich air-fuel ratio engine operation, the NOx trapping ability of the NOx trap catalyst is insufficient if NOx has remained in the NOx trap catalyst, thereby shortening a time in which the lean air-fuel ratio engine operation is made thus to lower fuel economy and exhaust gas purification performance.
It is, therefore, an object of the present invention to provide an improved air-fuel ratio control system for an internal combustion engine, which can effectively overcome drawbacks encountered in conventional air-fuel ratio control systems arranged in combination with a NOx trap catalyst.
Another object of the present invention is to provide an improved air-fuel ratio control system for an internal combustion engine, by which NOx can be sufficiently released from and reduced by a NOx trap catalyst under reducing materials in exhaust gas while totally lowering the emission levels of CO, HC and NOx with a good balance.
A further object of the present invention is to provide an improved air-fuel ratio control system for an internal combustion engine, by which oxygen remaining in a NOx trap catalyst and other catalysts disposed in an exhaust gas passageway of the engine can be sufficiently expelled prior to release and reduction of NOx in the NOx trap catalyst thereby optimizing reaction of NOx and the reducing materials (such as CO and HC).
An aspect of the present invention resides in an air-fuel ratio control system for an internal combustion engine provided with a NOx trap catalyst disposed in an exhaust gas passageway. The NOx trap catalyst is arranged to trap NOx in a first condition in which air-fuel ratio of exhaust gas flowing to the NOx trap catalyst is lean and to release and reduce trapped NOx in a second condition in which air-fuel ratio of exhaust gas flowing to the NOx trap catalyst is rich. The air-fuel ratio control system comprises a sensor for detecting an air-fuel ratio of exhaust gas in the exhaust gas passageway downstream of the NOx trap catalyst. Additionally, a control circuit is provided and configured to cause the engine to operate at a rich air-fuel ratio to accomplish a rich air-fuel ratio engine operation after an engine operation at a lean air-fuel ratio, and continue the rich air-fuel ratio engine operation for a duration even after the sensor has detected that the air-fuel ratio of exhaust gas is rich.
Another aspect of the present invention resides in an air-fuel ratio control system for an internal combustion engine provided with a NOx trap catalyst disposed in an exhaust gas passageway. The NOx trap catalyst is arranged to trap NOx in a first condition in which air-fuel ratio of exhaust gas flowing to the NOx trap catalyst is lean and to release and reduce trapped NOx in a second condition in which air-fuel ratio of exhaust gas flowing to the NOx trap catalyst is rich. The air-fuel ratio control system comprises first means for detecting an air-fuel ratio of exhaust gas in the exhaust gas passageway downstream of the NOx trap catalyst. Second means is provided to cause the engine to operate at a rich air-fuel ratio to accomplish a rich air-fuel ratio engine operation after an engine operation at a lean air-fuel ratio. Additionally, third means is provided to continue the rich air-fuel ratio engine operation for a duration even after the detecting means has detected that the air-fuel ratio of exhaust gas is rich.
A further aspect of the present invention resides in a method of controlling an air-fuel ratio in an internal combustion engine provided with a NOx trap catalyst disposed in an exhaust gas passageway. The NOx trap catalyst is arranged to trap NOx in a first condition in which air-fuel ratio of exhaust gas flowing to the NOx trap catalyst is lean and to release and reduce trapped NOx in a second condition in which air-fuel ratio of exhaust gas flowing to the NOx trap catalyst is rich. The method comprises (a) detecting an air-fuel ratio of exhaust gas in the exhaust gas passageway downstream of the NOx trap catalyst; (b) causing the engine to operate at a rich air-fuel ratio to accomplish a rich air-fuel ratio engine operation after an engine operation at a lean air-fuel ratio; and (c) continuing the rich air-fuel ratio engine operation for a duration even after the sensor has detected that the air-fuel ratio of exhaust gas is rich.